Tissue formation during development involves the determination, controlled proliferation and specific differentiation of cells in the embryo. Misregulation in any phase of this process can lead to failure in the development of the embryo, severe disease or uncontrolled cellular growth. Thus the study of gene regulation during development provides insight into areas important in human disease. Embryonic muscle formation in vertebrates and Drosophila (the fruit fly) provide excellent model systems in which to study the origin of one of the major tissues in higher organisms. The determination, proliferation, and differentiation of muscle cells during development in both vertebrates and invertebrates depend upon the function of the MyoD family of basic helix-loop-helix proteins, the muscle regulatory factors (MRFs). Determination of the first muscle precursor cells involves the activation of the MRFs in early mesoderm while gene expression characteristic of differentiated muscle remains repressed. Terminal differentiation is marked by the withdrawal of the myoblast from the cell cycle just prior to the activation of the muscle-specific genes and both processes involve the MRFs. Furthermore, greater than 90% of the genes expressed in the dividing muscle cell are shut down in a process that involves massive chromatin reorganization while the muscle-specific genes are activated. Cell cycle control during terminal differentiation is thought to involve the MRFs in a pathway that regulates the phosphorylation status of the retinoblastoma protein, Rb. (Project 1) We have recently shown that ectopically expressed MyoD binds directly to the G1 cyclin-dependent kinase cdk4 to inhibit cell growth and the phosphorylation of Rb. The cdk4-MyoD interaction also blocks the trans activation functions of MyoD by disrupting DNA-binding by the MyoD/E-protein heterodimer. Therefore, high levels of nuclear cdk4 block MyoD function in growing myoblasts while the loss of nuclear cdk4 in the absence of growth factors and mitogens allows MyoD to function. We have identified a 15 amino acid domain on MyoD responsible for the interaction with cdk4. Expression of this domain either as a fusion protein with GST or GFP inhibits the kinase activity of cdk4 in vitro and in vivo, blocking its ability to phosphorylate the retinoblastoma protein, Rb. This results in the cessation of cell growth and induces myoblast differentiation in the presence of mitogens. We have a patent application on the inhibitory activity of the 15 amino acid domain of MyoD on cdk4 kinase activity. We have recently made alanine substitutions in all the positions of the 15 amino acid cdk4-binding domain in order to map the critical residues for interaction. Single substitutions have a marginal affect on inhibitory and binding activity of the domain but two simultaneous substitutions reduce cdk4 binding and kinase inhibition for the various binding domain derivatives. The binding parameters are being determined uisng the BiaCore and the imobilized 15 amino acid derivatives. We have also determined that the MyoD 15 amino acid domain binds to the other major G1 cyclin-dependent kinases, cdk6 and cdk2. cdk6 behaves like cdk4 during muscle differentiation in that cdk6 leaves the nucleus when mitogen levels are reduced but can be induced to re-enter myotube nuclei with the expression of a stable cyclin D1 in the cells. cdk6 phosphorylation of Rb is also inhibited by the MyoD binding domain. However, although cdk2 binds to the same 15 amino acid domain, phosphorylation of histone in vitro is not inhibited. All the in vitro kinase assays are performed using baculovirus produced cyclin D1/cdk4, cyclin D1/cdk6, and cyclin E/cdk2 purified by Flag-tag affinity chromatography. cdk4/6 kinases are inhibited by p16 and p21 while cdk2 activity is only blocked by p21.